electricity

Basic Electrical Concepts

 

Charge

 Electricity is a rather multi-faceted subject and a good understanding of the basics is important. We begin with a “SCI200″ review of charge. Charge is a basic property possessed by electrons and protons. Charge comes in two types, negative (the type on an electron) and positive (the type on a proton.) Opposite charges attract and like charges repel one another. Atoms are formed by light, negatively-charged electrons orbiting around positively-charged protons in the much heavier nucleus. In general, matter has equal numbers of protons and electrons and is neutral overall. An object becomes charged only when electrons are added to or stripped away from the object.

Current

Generally, a current is any movement or flow of charge. In household applications, it is specifically the movement of electrons through wires and electrical devices. There are several factors that determine the flow of current. First, we classify all substances into two broad categories, according to how well current can flow through them.

Insulators are substances in which all the electrons in the atoms of the substance are tightly bound. The electrons do not easily move from one atom to the next. It is very difficult to get current to flow through insulators. Examples of insulators include: ceramics, rubber, plastic, and surprisingly, pure water.

Conductors are those substances in which the outer electrons (typically only one or two in each atom) can move freely from one atom to the next. Current flows easily through conductors. Examples of conductors include: metals such as copper, iron, aluminum, and water‹that contains a lot of dissolved minerals.

Although most substances are easily classified as one or the other, there is no absolute dividing line between insulators and conductors. There are substances which lie somewhere in between. Semi-conductors, important for electronics, are an important example. The glowing filaments in light bulbs and toasters are often classified as conductors, but they are actually not very good conductors. As we will see below, they shouldn’t be. The electrical wiring in the walls of your home and the power cords for electrical devices, such as lamps and refrigerators, are much better conductors.

Consider the diagram of a conducting wire, in which a few of the outer electrons are shown. A current (denoted I) exists in the wire as the electrons move along the wire. (Note that the wire stays neutral as current flows. As one electron jumps to a neigboring atom, another moves in to take its place.) The more charge that passes per unit time, the greater the current. The idea is simple. Current is a measure of the number of electrons that flow past any point along the wire during any time interval, divided by that time interval.

I    =    ^charge / _time

The units of current are amperes or just amps (denoted A). One amp represents a flow of about 6 x 10¹⁸ electrons per second! Is one amp a lot of current? Despite the incredible number of electrons per second, one ampere is roughly the amount of current that flows in the common household 100 Watt incandescent light bulb. It is also roughly the amount of current that flows in small flashlight. Are you surprised? There’s more to learn.

Electric Potential

Electric potential is what drives current. You may know electric potential by another term that we will use … voltage. This name comes from the unit of potential, which is the volt (denoted V). When you buy an AA battery, you are buying a device that provides a potential of 1.5 V between its positive and negative terminals. Your car battery maintains about 12 V between its terminals. And the potential between the two slots in a household electrical outlet is about 120 V. (Although not important here, the different nature of the potential of the outlet will be considered in the AC section.) You are probably already familiar with a basic truth about electric potential. All other things being equal, a greater potential will create a greater current. But what is electric potential?

 Water can provide a good analogy (although far from perfect!) for both current and potential. Consider a pipe that comes out of the bottom of a large tank of water, such as shown above. You open the spigot and water flows. The flow rate of the water is analogous to current. Common sense tells you that the higher the water level in the tank, the higher the flow rate in the pipe. (We will investigate this further in the Plumbing module!) The height of the water level is analagous to electric potential. A greater potential will cause a greater current.

Where this analogy fails is with the battery. The tank stores water and as the height slowly decreases, so does the water flow. A battery does not store charge! It is always electrically neutral and for whatever amount of charge leaves one terminal, an equal amount must come into the other. (As we will see in the next section, a complete circuit is required for this to happen.) A battery is more analogous to the water pump shown in Figure 1-3. A battery, therefore, is an electron pump! It has the ability to push electrons directly proportional to its voltage rating. And, it does this through a chemical reaction. The battery becomes “discharged,” (an unfortunately misleading term), when the chemicals in the battery are used up. Most batteries maintain a fixed potential until near the end of their life. The 120 V potential of a household outlet is produced in a very different way. There will be more on this topic in an upcoming section.

Resistance

How much current flows when a given potential is present? That depends upon the resistance to the flow of charge. For a given potential, low resistance results in a higher current and high resistance results in a lower current. The resistance of an object depends upon both the material used and it’s shape. A good conducting material has lower resistance while an insulating material has higher resistance. A long wire has more resistance than a short one. A thick wire (having a large cross section) has less resistance than a skinny one.  Resistance (R) is actually defined by the ratio of potential (V) to current (I).

R    =    ^V / _I

The unit for resistance is the volt/amp, called an ohm, and is denoted by the greek symbol omega (W). Associated with this definition is Ohm’s Law, which is represented by the same equation, but usually written as V = IR. We will use Ohm’s Law in the next section on circuits.

Resistance and Heat Energy

 Resistance in a material arises from the collision of electrons with the atoms and with each other as they move. The collisions produce heat, increasing the temperature of the material. Consider the ordinary toaster shown* in Figure1-4. Current flows through the wires of the power cord and through the toaster’s filament (the glowing wire you see inside). The same current must flow in the power cord as flows through the filament. The cord has very little resistance, while the filament has considerably more. Since the filament has a much higher resistance than the cord, it produces much more heat. That’s as it should be. You want the heat for your toast, but you do not want the power cord getting hot! The standard incandescent light bulb is another example. The filament in the light bulb glows white hot (hence, the word “incandescent”) to produce light and a lot of heat as well. But, the low-resistance power cord stays cool.

Toasters and light bulbs are called resistive devices. They convert electrical energy into heat and light energy. Electrical devices with motors, such as refrigerators or blenders, are more complicated than simple resistive devices. They are designed to convert elecrical energy into mechanical energy. (We will study this in more detail later.) Nevertheless, they have an effective resistance. In general, the power cords and electrical wiring in your home should have much less resistance than the devices to which they supply current. Power cords and electrical wiring are rated by the maximum current they can carry without significant heating. That brings us to another important concept.

Power

How much energy does your toaster use? That depends upon how many pieces of bread you toast. Devices are not rated by the energy they consume, but by the rate at which they consume energy, the power.

Power is energy per time. The standard unit used in electricity is the Watt (W) = 1 Joule / second. (Need a review of energy?) A 100 W light bulb will consume 100 joules of energy every second that it is in operation. Batteries (and WAPA) supply power, while electrical devices such as light bulbs and refrigerators consume power. There is a simple relationship between power and all the other quantities we have discussed so far. For all electrical devices, the power that they supply or consume is the product of the potential across the device and the current that flows through the device.

P = I V   all devicesIf a device has a well-defined resistance such as a light bulb or resistor (a device purposely designed to limit current), we can also use our expression V = I R to get

P = I² R   resistorsIn this form, we can see more clearly the importance of the current in power consumption. If we were to double the potential across a resistor, the current would also double (V = I R). But the power consumption of the resistor would increase by a factor of 4! We’ll see the importance of this later when we consider energy losses in high voltage powerlines

PHARMACY/ high blood pressure

High Blood Pressure: Getting the Most Out of Home Monitoring

Victor J. Lewis, PharmD, and Robert Lee Page II, PharmD, FASCP, CGP, BCPS

Dr. Lewis is a pharmacy practice resident at University of Colorado (UC) Hospital. Dr. Page is an associate professor of clinical pharmacy and physical medicine and a clinical specialist, Division of Cardiology, UC Health Sciences Center, Schools of Pharmacy and Medicine.

Blood pressure is often used to predict the risk for several serious health problems,such as heart disease. Many people are used to getting their blood pressure checked regularly at their doctor’s office. While quick and easy to measure, blood pressure can vary greatly throughout the day, depending on a patient’s surroundings, physical well-being, recent meal, or even the time of day.

These factors make it difficult for the doctor to understand how to treat the high blood pressure (also called hypertension) since he or she only measures the blood pressure during appointments. Fortunately, a wide variety of home blood pressure monitors can be purchased at pharmacies so that patients can take an active role in helping control their blood pressure by checking it at home.

Why Should I Monitor My Pressure at Home?

There are several advantages to monitoring your blood pressure at home. First,it creates a more complete picture of what your blood pressure looks like throughout the day.

Taking your blood pressure at home, gets rid of the stress of being at the doctor’s office. When the blood pressure is increased due to the anxiety of being in the doctor’s office, this is called white-coat hypertension.

Second, monitoring blood pressure at home will help you see if your medications or changes in your lifestyle such as a low-salt diet or increased exercise are working to keep your blood pressure down. Keeping a list of your blood pressure measurements at home helps your doctor make better decisions about your care. Finally, home blood pressure monitoring has been shown to result in better blood pressure control and greater success in getting your blood pressure under control. This could mean fewer doctor visits.

There are times, however, when home blood pressure monitoring may not be recommended. For example, most home blood pressure monitors may not provide correct readings if your heartbeat is too fast or too slow. If you are overweight or very muscular, a regular-sized blood pressure cuff may not fit around your arm and could provide incorrect readings.

Therefore, you might need a large cuff, which is usually sold separately, or you can purchase one of the new preformed expandable cuffs that fit regular and large arms, such as the ComFit cuff from Omron. Your pharmacist or doctor can help you decide if home blood pressure monitoring is right for you.

What Are the Different Types of Home Monitors?

Several different blood pressure monitors are available at your local pharmacy. The range of choices include manual and automatic devices. For the first-time user, this may be confusing, so you should ask your doctor or pharmacist what type of monitor is best for you.

Home blood pressure monitors are either manual or automatic devices. Some manual devices, referred to as self-taking, consist of an inflatable cuff and a stethoscope attached to a gauge by a rubber tube. The gauge will most often be a dial with a needle corresponding to the current pressure. You must place the cuff on your upper arm with the stethoscope positioned over the brachial artery, which is located slightly above the inside of your elbow. The brachial artery is a large artery that runs from the shoulder down to the elbow.

By pumping the bulb, you will inflate the cuff. With the stethoscope, you can listen to certain blood vessel sounds and your heartbeat as you slowly deflate the cuff. Your pharmacist can help you with the instructions. Other manual devices use more advanced technology that automatically determines your blood pressure reading, eliminating the need for the stethoscope. These manual devices include an inflatable cuff, a manual pump, and a monitor that provides a digital readout.

Most patients prefer using an automatic device, which consists of a cuff and a monitor that records the pressure automatically. First, you place the cuff on the upper arm. Second, by simply touching a button, the device automatically inflates and deflates the cuff while measuring the blood pressure and heart rate.

Many of these devices can detect an irregular heartbeat, have an internal memory function that can store and recall your blood pressure readings, and automatically calculate and display the average of your most recent readings, and some even help determine if you have morning hypertension, which is a significant risk factor for stroke. Morning hypertension is when your blood pressure is too high in the morning.

Wrist blood pressure monitors that automatically inflate when the wrist is at heart level have been clinically proven as accurate as upper-arm devices. However, finger devices are not accurate and should not be used.

Can I Rely On Public Blood Pressure Machines?

You should not rely on public blood pressure machines found in grocery stores or malls. These machines are often not checked on a regular basis for accuracy.

What Should I Consider When Purchasing a Monitor?

Many factors should be considered when purchasing a home blood pressure monitor. The most important is choosing a monitor that has been validated for accuracy. Check with your pharmacist to ensure that the monitor has been clinically proven to be accurate. The next most important feature is having a monitor with a cuff that fits your arm. Your pharmacist can help determine what size cuff is needed specifically for you. Larger cuffs may be slightly more expensive, but ones that are too small will give incorrect readings.

Make sure you can read the numbers on the display screen clearly and understand how to use the device before buying. Consider cost when shopping for a home blood pressure monitor. The devices average around $79. However, they start at about $20 for a manual self-taking unit with a stethoscope and range to just over $100 for a fully featured model with computer software that allows you to download, track, and print your readings from your home computer.

How Do I Take Accurate Blood Pressure Readings at Home?

Now that you have purchased your monitor, you may want to ask your doctor to watch you take a blood pressure reading to make sure you are applying the cuff correctly. There are several important things you should remember before using the device at home.

Check your blood pressure at the same times every day, about an hour after you wake up and in the evening, about an hour before bedtime. Always use the same arm each time. Make sure that you are in a quiet, comfortable setting when taking your pressure. Avoid caffeine, food, tobacco, and alcohol for 30 minutes before checking your blood pressure. A full bladder can increase blood pressure slightly, so go to the bathroom first.

Before taking your pressure, sit in a comfortable position with your back supported, legs and ankles uncrossed, and feet on the floor for at least 3 to 5 minutes. Do not talk, eat, or chew gum while taking your blood pressure. Place the cuff directly on the skin, and do not roll up long sleeves. This can make the readings incorrect. Take a second blood pressure measurement 1 to 2 minutes after the first one is finished for better accuracy. If your device does not have memory, write down your blood pressure readings, including the date and the time of day they were taken. Bring the record of your blood pressure readings to all doctor appointments. If you have questions, ask your pharmacist or doctor.